A1 = mole fractions of hydroxyl groups on monofunctional moieties A2 = mole fractions of hydroxyl groups on difunctional moieties A3 = mole fractions of hydroxyl groups on trifunctional moieties a = empirical constant b = empirical constant C1 = intercept C2 = slope d = relative contents of difunctional moieties Eb = elongation at break ENCO = number of equivalents of diisocyanate compound EOH = number of equivalents of hydroxyl groups E0 = modulus fs = relative equivalent fraction of secondary hydroxyl groups in the curing polymer mixture k = rate constant for viscosity buildup k1 = rate constants for viscosity buildup for stage 1 k2 = rate constants for viscosity buildup for stage 2 Lsp = number of chain atomsper gramof the polymer network Lx = effective chain length m = relative contents of monofunctional moieties p = extent of cure reaction pNCO = extents of reaction on NCO groups pOH = extent of reaction on OH groups R = universal gas constant r = stoichiometric ratio T = temperature t = relative contents of trifunctional moieties ti = time at the point of digression Vs = molar volume of the solvent in which the polymer is swollen νE0 = crosslink density calculated from Young’s modulus νe = crosslink density, mol∕m νp = physical entanglements νte = concentration sliding-type chain entanglements v2 = volume fraction of polymer in a swollen elastomer specimen Weq = equivalent weight WG = weight fractions of gel Ws = weight fractions of solution X = fraction of urethane groups involved in the allophanate formation xi = extent of reaction at the point of digression α = branching coefficient ηi = viscosity at the point of digression